Well it's not the same size as your desktop, or anyone's for that matter. You will either have to zoom in and have parts cut off from view or you will have all of it centered with a colour, hopefully black surround it on your desktop.
Now getting a wallpaper that is the size of your desktop gets rid of all that. There are some wallpaper sites that tell you what size your desktop is if you don't know and don't know how to find it.
Here is one that does that without anything else around to ... clutter it. [link]
As for the smile, I have a few more for you. [link]
You can resize it. But making it bigger will make it a little fuzzy, and you will still have to cut some of it off if you want it to fill your whole screen.
You have to use ':' one either end. Second you don't have any spaces and lastly you have to use icon at the start of each one. In the comment I posted you see all the words and what it should look like. This is when you start to use the preview button to check your reply before sending it to make sure the face you shows up.
Next you have to say icon to start off all code and end with plz.
Once you know that you just have find out what goes in between. That normally tells you what it's going to be.
:icon grin-- plz: lets you know that it's a grin image. The -- means that there are other ones out there or it would just be :icon grin plz:
Now if you take out the spaces you will see this with out the -- it will look like this with one - you get this
Do you get it?
If do or don't, this should help you out. One, if you do get it, you can find all kinds of faces and such here to use, and two, if you don't get it, you just copy the 'code' under the one you want to use and paste it in your message.